The HIV-envelope protein gp120 plays a critical role in triggering the apoptosis of T-lymphocytes, which is a central pathogenic feature of HIV-mediated immune dysfunction. T-cell apoptosis is also a physiological process that regulates antigen receptor repertoire selection during homeostasis and the maturation of T- cells. We hypothesize that gp120-induced cross-talk between T-cell receptor (TCR) and chemokine receptor signaling pathways leads to apoptosis. Our preliminary data on the role of the TCR signaling components CD45 and SLP-76 in HIV-gpl20-induced apoptosis support this hypothesis. In addition, we have recently shown that cross-talk between TCR and chemokine receptor CXCR4 regulates apoptosis via a novel mechanism that is mediated by AKT/Protein Kinase B (PKB), heat shock protein-70 (HSP-70), caspase-1 and RIP2 (caspase recruitment domain-containing serine/threonine kinase). To analyze mechanistically how the TCR components and chemokine receptors CXCR4/CCR5 regulate gp120-induced apoptosis, we will pursue the following specific aims: Aim 1) we will assess the interaction between CD45 and CXCR4/CCR5 signaling molecules by defining the domain of CD45 and characterizing the CD45 signaling responsible for apoptosis. Aim 2) we will perform structural and functional analyses of the TCR-mediated downstream effector SLP-76 that mediates Ca2+dependent apoptotic pathways. Aim 3) we will analyze the role of the AKT/HSP-70 apoptotic pathway that may lead to the induction of forkhead transcription factors, and will characterize the activation of caspase-1 and its regulation by RIP2. We are also exploring innovative strategies to inhibit gp120-induced apoptosis. In this regard, we have shown that a novel protein, Slit, which binds to the Robo receptor and modulates CXCR4 function can inhibit gp120-induced apoptosis. Aim 4) we will identify which domains in the Slit/Robo complex regulate the observed anti-apoptotic effects. These studies are designed to identify apoptotic signaling molecules and effector pathways involved in T-cell loss, and thus to provide novel therapeutic targets to combat immune deficiency in AIDS.